Single-crystal silicon is used for most electronic applications. Exceptions exist, such as displays and some imagers, where amorphous silicon is applied to glass substrates in order to operate the display or imager pixel. In many applications, the display or imager is fabricated on top of the silicon electronics. For application to liquid crystal displays (LCDs), amorphous silicon has provided sufficient performance. For next generation display devices such as Organic Light Emitting Diodes (OLED), active matrix drive transistors made from amorphous silicon have proven problematic. Fundamentally, LCDs use voltage devices, and active matrix OLEDs require current devices. Attempts to extend the conventional approach involve modifying the prior-art amorphous-silicon on glass. Amorphous-silicon is applied to the entire substrate panel, typically greater than two meters on a side, and then is re-crystallized using large excimer lasers and scanning a line focus across the panel. The laser has to be pulsed so as to only melt the Si surface and not the glass. This technique results in the formation of poly-crystal silicon rather than single-crystal silicon.
The mobility of any type of amorphous or poly-crystalline transistor, including non-silicon and organic devices, is much smaller than the mobility of single-crystal silicon transistors. Electron mobility in amorphous silicon is ˜1 cm2/V·s compared to ˜100 cm2/V·s for poly-silicon, and ˜1500 cm2/V·s for high-quality single-crystal silicon. It is therefore advantageous to use single-crystal silicon in place of amorphous silicon in such devices. However, silicon wafers are typically 300 mm in diameter, compared to current display panels which can measure at more than 2 meters on a side. In the case of such large area devices, including large area OLED displays, larger wafers of single crystal silicon can become prohibitively expensive and/or technically impractical to fabricate.
In addition, in making conventional active matrix OLED displays the light emitting material is typically deposited as a thin film on an active matrix backplane. This process has at least two significant limitations: first, even a small degree of roughness on the surface of the backplane can interfere with the deposition of the light emitting layers and cause malfunction in the final display. Second, since the light emitting layers and the backplane are rigidly attached to one another, there must be a close match between their respective coefficients of thermal expansion (CTE) to avoid damage to the display as a result of temperature changes. This need for a close CTE match limits the types of materials that can be used for the backplane and for the light emitting layers.